Exploration for underground resources such as hydrocarbons includes the evaluation of reservoirs to determine the presence and/or movement of fluids such as oil, gas, and water. One method for such an evaluation includes the use of measurements obtained using a downhole tool. For example, gravity measurements may be used to map out the vertical distribution of oil and gas in a borehole by enabling the calculation of the bulk density of an area adjacent to the downhole tool based in part on relatively minute gravity changes between different positions of the borehole.